Controlled engine exhaust gas recirculation is a commonly-used technique for reducing oxides of nitrogen in products of combustion that are exhausted from an internal combustion engine to the atmosphere. A typical EGR system comprises an EGR valve connected either to the exhaust manifold or the intake manifold that is controlled in accordance with engine operating conditions to regulate the amount of engine exhaust gas that is recirculated to the induction air flow entering the engine for combustion so as to limit the combustion temperature and hence reduce the formation of oxides of nitrogen.
Since they are typically engine-mounted, EGR valves are subject to a harsh operating environment that includes wide temperature extremes and vibrations. Exhaust emission requirements impose more stringent demands for improved control of such valves. Use of an electric actuator is one means for obtaining improved control, but in order to be commercially successful, such an actuator must be able to operate properly in such extreme environments for an extended period of time. Moreover, in mass-production automobile vehicle applications, component cost-effectiveness is also essential. An EGR valve electric actuator that possesses more accurate and quicker response results in improved driveability and fuel economy for a vehicle having an internal combustion engine that is equipped with an EGR system. It also provides better control over tail pipe emissions.
One problem with the EGR valves is their ability to accurately control the amount of exhaust gas flow over a wide range of operating conditions. Many EGR valves have a substantially linear response over their entire range of opening. To provide accurate control, however, a higher degree of positioning accuracy is required at low flow rates than at high flow rates. Until now, this capability was limited due to the common construction of the EGR valves. As a result, it has been difficult to meter precise small amounts of exhaust gas through the EGR valve.
This is especially difficult for electrically operated EGR valves (EEGR valves) that depend upon linear electrically operated solenoids to open the valve. An example of such a valve is shown in U.S. Pat. No. 5,911,401, which is incorporated herein by reference for all that it teaches.
In the '401 patent, the EEGR valve comprises an elongate valve pintle having a tapered outer surface that is moved by an electrical solenoid towards and away from an annular valve seat. This operation provides a single valve opening with a flow area that varies proportional to the distance the valve pintle moves. As a result, the valve curve has a constant slope, and the valve error and lack of precision is substantially constant over the entire operating range of the valve.
What is needed, therefore, is an improved EGR valve having an improved valve response curve with lower error and higher resolution when the valve is almost closed. It is an object of this invention to provide such an EGR valve.